1. Field of the Invention
This invention generally relates to a movable robot of, for example, a self-moving type. This invention specifically relates to a wheeled robot having three rotation axes or axles.
2. Description of the Related Art
Japanese patent application publication number P2001-322079A discloses a humanoid robot or a bipedalism robot having a body equipped with various sensors. The robot body is divided into portions connected by joints which can be driven by actuators. The sensors include gyro sensors and acceleration sensors mounted on the robot body, and encoders located near the joint actuators. The gyro sensors, the acceleration sensors, and the encoders compose a body-posture sensing arrangement. An action/posture management section in the robot operates the joint actuators in response to the output signals of the gyro sensors, the acceleration sensors, and the encoders to properly control the posture of the robot body.
Japanese patent application publication number P2000-218578A discloses a globe-shaped movable robot which has a spherical shell and a main unit disposed in the spherical shell. The main unit includes a first wheel, a second wheel, a first motor for driving the first wheel, and a second motor for driving the second wheel. The first and second motors can be operated by a motor controller in the main unit. The first and second wheels are axially spaced from and parallel to each other. The outer circumferential surfaces of the first and second wheels are in contact with the inner surface of the spherical shell. In the case where the motor controller operates the first and second motors to rotate the first and second wheels at equal speeds in a same direction, the spherical shell moves forward while rotating in a direction opposite to the direction of rotation of the first and second wheels. In the case where the motor controller operates the first and second motors to rotate the first and second wheels at equal speeds in opposite directions respectively, the spherical shell spins about a vertical axis while the center of the shell remains substantially at a same point. In the case where the motor controller operates the first and second motors to rotate the first and second wheels at different speeds respectively in a same direction, the spherical shell turns along a curved path.
Japanese patent application publication number 9-254838/1997 discloses a globe-shaped movable body which includes a spherical shell and a square base disposed in the shell. The square base extends horizontally. Three driving wheels and a caster (an idler wheel) are provided on the four corners of the square base, respectively. The driving wheels and the caster are in contact with the inner surface of the spherical shell. The driving wheels can be actuated by motors, respectively. As first one of the driving wheels is rotated by the related motor, the spherical shell moves in a pitch direction. As second one of the driving wheels is rotated by the related motor, the spherical shell moves in a roll direction. As third one of the driving wheels is rotated by the related motor, the spherical shell moves in a yaw direction. When two of the driving wheels are simultaneously actuated, the spherical shell makes a composite movement.
The bipedalism robot in Japanese application P2001-322079A has an extremely complicated structure and a very large number of parts. The gyro sensors, the acceleration sensors, and the encoders in the bipedalism robot are expensive and large in size. Therefore, the bipedalism robot tends to be high in cost and great in size. Accordingly, the bipedalism robot is unsuited for home use. In the bipedalism robot, the body-posture control is based on a very complicated algorithm. Generally, such an algorithm necessitates the use of a special computer which can process data at a high rate. Since the size of a computer program for such an algorithm is large, a memory related to the computer needs to be huge in capacity. The special computer and the huge-capacity memory are expensive. The bipedalism robot has a considerable chance of falling down when meeting an obstacle.
The globe-shaped movable robot in Japanese application P2000-218578A and the globe-shaped movable body in Japanese application 9-254838 tend to be unable to maintain their correct postures and are liable to spontaneously move down when they are on a sloping floor. Since the spherical shells of the movable robot and body remain in contact with floors, the outer surfaces of the shells tend to become dirty and flawed as a result of long-term use. The movable robot and body can not make complicated movements and quick movements. Accordingly, the performances of the movable robot and body are insufficient for home use.